Strange to think that the fulmar once had only a single breeding station in all Britain – St Kilda – until it embarked on a slow expansion around our coasts that has eventually taken it to Kent and Cornwall. It probably arrived at this place in the early 20th century, where it showed astonishing adaptability.

Previously the birds had expressed a preference for sheer cliffs as nesting locations, but this low-lying island only rises to 75ft. Its one striking feature, in fact the largest artefact in all Orkney, is a sea wall (the "sheep dyke") that bounds the island's entire shore and was built to keep its seaweed-eating sheep off the land and on the rocky beaches.

The 180-year-old drystone wall runs for 12 miles and is now so mottled with the exquisite soft green of sea ivory and the bright orange from parietina lichen that it is a living organism by itself.

The dyke represents millions of hours of hard human labour, but to the fulmars it is the most sheltered spot to lay. At intervals in the earth, buttressed at their rear by the dark stone, the parent birds sit patiently on one huge ivory egg or squat with the oil-filled bag of white fluff that is the growing chick.

Together, parent and offspring create little pale globes of softness in the wall's shadow. The other adults, many of them the mates of the sitting birds or non-breeders, use the dyke as a plaything and alternate sailing down into the lee of the wall with a sweet rise and turn to catch with perfect timing the uplift of the breeze.

Wherever you go, these patrolling fulmars cruise back and forth along the line of the dyke, cutting the island's renewed template from the clean air and shaping its character until you cannot think of North Ronaldsay without them.

They are the geography of this hard, salt-fashioned place, turned into feather and air.